Repeated attempts have been made in the prior art to provide a machine or apparatus capable of successfully welding the face sheets of a panel which incorporates a metallic core to said core.
Numerous machines and methods have been disclosed in the prior art but the most commercially successful machines and apparatus known to the applicants are disclosed in Campbell U.S. Pat. Nos. 3,689,730; 3,598,953; 3,077,532; 3,015,715; and 2,930,882. The various embodiments of the machines and apparatus disclosed in the aforementioned patents revolve around the concept that the weldment of securement portions of core strips to inner surfaces of cooperating face sheets to fabricate a metallic panel is accomplished by the utilization of externally positioned, movable welding rollers or wheels which cooperate with rigid projection welding bars or electrodes disposed internally of the core strips and face sheets.
As previously mentioned, considerable commercial success has been achieved by the utilization of the Campbell apparatus and machines in fabricating stainless steel and titanium core reinforced metallic panels. However, the utilization of the relatively massive welding wheels and rollers in conjunction with the rigid projection welding electrodes entails the flow of welding potential through the relatively thick face sheets into the relatively thin securement portions of the core strips to create a welding current path with the relevant projections of the projection welding electrodes. Consequent upon such techniques and inherent in the utilization of the prior art machines and apparatus has been the necessity for high pressures on the order of 20,000 psi to 30,000 psi and high amperage currents on the order of 2000-6000 amps peak.
The welding wheels of the prior art contact the outer surfaces of the face sheets and sequentially distribute the electric pulses to the face sheets as they traverse the panel width, but all the projections on the welding electrodes are in electrical contact with the core securement portions and the inner surfaces of the face sheets with the result that large amounts of weld current are bypassed through the zones not being welded and large pressures are required in the zone being welded in order to define the weld zone.
Stainless steel and titanium core reinforced panels manufactured on the prior art Campbell machines have been widely and successfully utilized in the aircraft industry in applications where high temperatures and sonic fatigue have been encountered, such as thrust reverser doors for jet engines. However, the prior art panels are characterized by the presence of welding "nuggets" of excessive size due to the high pressures and the relatively high amperages which must be utilized to project the welding potential through the relatively thick face sheets and through the relatively thin securement portions of the core strips to create a weld zone with the projections of the rigid projection welding bar that react against the massive welding wheel pressure, while other projections bleed off welding current.
It is characteristic of prior art panels that a conspicuously perceptible welding path may be discerned across the external surfaces of the face sheets, said welding pattern being attributable to the excessive weld nugget size created by the previously mentioned pressure-amperage parameters encountered in the utilization of the prior art apparatus.
We have found that, from a physical and structural analysis of the prior art panels, there is absolutely no necessity for the utilization of the high pressures and high amperages entailed by the prior art apparatus or machines to securely fasten the securement portions of the core strips in welded and operative relationship with the interior surfaces of the face sheets. Additionally, this prior art apparatus limits the thickness of the face sheets that may be used.
As a matter of fact, physical experiments have demonstrated that a series of relatively small weldments is capable of achieving the permanent affixation of the securement portions of the core strips in operative engagement with the internal surfaces of the cooperating face sheets.
Surprisingly enough, we have also found that, as the size of the welding "nuggets" is reduced, it is possible not only to create a greater number of weldments in a series but to achieve structural characteristics in the panel not attainable by the use of prior art welding machines subjected to the inherent demands of pressure and amperage previously described.
Of course, the prior art machines cannot be utilized to fabricate panel by the generation of the limited welding current pulses and pressures necessary to curtail the size of the "nuggets" or weldments which have been the subject of our experimental endeavors. Consequently, we have invented a machine or apparatus as succinctly set forth in the preceding abstract of the invention and disclosed hereinbelow.